Adenosine triphosphatase (ATPase) was studied in tissue homogenates and subcellular fractions derived from human cadaver kidneys maintained in an organ preservation unit for transplantation. The activity of ouabain-sensitive ATPase was highest in the medulla, intermediate in the cortex and lowest in the papilla, The cortical enzyme activity diminished with time during maintenance perfusion of the kidneys. Similar concentrations of K+, Na+, Mg++, ATP and MgATP were required for half-maximal rates of ouabain-sensitive ATPase activity from the cortex or the medulla. The sensitivity of the enzyme to ouabain from both parts of the kidney was similar. K+ antagonized inhibition of the enzyme by ouabain. Chlormerodrin, mersalyl, mercaptomerin and ethacrynic acid were inhibitors of the enzyme.